Generally, a noise reduction device is provided for an engine to generate interference sound having an opposite phase to noise caused in an air suction system of the engine, so as to reduce air suction noise of the engine. In this case, it is necessary to generate interference sound in synchronization with the rotation of the engine, so that considerable electric power and a corresponding control apparatus become required.
Referring to JP-2004-245090A, there has been proposed an air-suction noise reducing device which has an air vibration exciting apparatus for vibration-exciting air responding to the operation of the engine.
In this case, air having a varying pressure is supplied for the air vibration exciting apparatus, to vibration-excite air in a suction passage in order to reduce air suction noise of the engine. However, the air pressure is switched by a switching valve, it is difficult to precisely control an amplitude of the air pressure provided for the air vibration exciting apparatus corresponding to noise caused in the air suction system. Moreover, because the switching valve is controlled by a controller, it is difficult to provide the varying pressure of air in synchronization with the rotation of the engine.